A 12-month-old child is brought to the immunisation clinic for the 12-month vaccination visit as per the National Immunisation Schedule. The mother reports that the child had severe allergic reaction (urticaria and angioedema) after the first dose of DPT vaccine at 6 weeks. The child is otherwise healthy and has received all other scheduled vaccines without adverse events. Which investigation is most appropriate to determine if the child can safely receive the second dose of DPT or if an alternative vaccine formulation is required?